Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing our new design.

The dotted and evenly-dashed broken lines, specifically, a 5-by-5 array of circles and an icon and text in the bottom left of the banner, illustrate portions of the graphical user interface that do not constitute part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 